A photopolymerizable composition fundamentally comprises a photopolymerization initiator and a polyfunctional monomer, that is hardened upon being irradiated with light to become insoluble in solvents. Such compositions are widely utilized in photography, printing, surface processing of metals, inks, etc. (see J. Kosar, Light-Sensitive Systems, 158-193 (J. Wiley & Sons, New York, 1965)).
Studies on photopolymerizable compositions have been made to enhance their sensitivity to light, and many photopolymerization initiators have been proposed to achieve the end; for example, benzoin ethers described in U.S. Pat. No. 2,448,828, benzoins described in U.S. Pat. No. 2,722,512, anthraquinones described in U.S. Pat. No. 3,046,127, aminophenylketones and active methyl or amino compounds described in Japanese Patent Publication No. 11936/74 (corresponding to U.S. Pat. No. 3,661,588), Michler's ketone and benzophenone described in U.S. Pat. No. 3,682,641, and benzophenone and 4-N,N-dimethylaminobenzophenone described in Japanese Patent Publication No. 38403/73 (U.S. Pat. No. 3,549,367). These photopolymerization initiators improve photosensitivity to some extent, but the resulting photo-hardened products (particularly film-like hardened products) have mechanical properties that may be insufficient for various end uses.
For example, when used as dry film resist in preparation of printed-wiring boards, they provide insufficient film strength after photo-hardening. Japanese Patent Publication No. 25231/70 describes dry film resists for preparing printed-wiring boards, and more detailed descriptions are given in, for example, W. S. De Forest, Photoresist, 163-212 (McGraw-Hill, New York, 1975). The main use of dry film resists is for preparation of perforations by tenting. However, conventionally known photopolymerization initiators have the disadvantage that due to insufficient strength of the tenting film, film breakage occurs in developing and etching steps.